octane and mpg

Marc Swanson mswanson@sonitrol.net
27 May 2003 15:17:01 -0400

> I'm back on the q-list after an absence of a couple of years.  My
> curiosity got the best of me, I guess, and the boy racers over on the
> AudiWorld forum are a bit much to take.

welcome back, Shaun.

> I have a question right out of the gate:  Is there a relationship
> between octane and miles per gallon obtained?  In other words, and
> understanding there probably are other factors, will I get better gas
> mileage running my A4 on 93 octane than 89 octane?

The answer is yes, maybe :-)

If you were to drive the car under identical conditions with an even
foot between a tank of 89 and 93  then the tank of 93 SHOULD give you
slightly better gas mileage.

There was a thread not too long ago regarding the use of toluene as an
octane booster, and lister Ron Wainwright experimented with this in a
normally aspirated 5000 and logged better mileage with high octane
fuel.  The reason for this is that the higher octane fuel is capable of
releasing more energy per unit and thus the ECU will meter a smaller
amount of it under identical conditions vs. a lower octane fuel.

If your A4 is a 1.8t (and maybe even on the NA 2.8) your ECU will detect
the fact that you are running a higher octane fuel and may change your
fuel and ignition maps a bit to give you more power which can in turn
increase fuel consumption, likely nulling out any increase in fuel
economy you may have attained from going with higher octane fuel.

Best Regards

87' 4ktq
88' 90q